Heretofore, in large-sized boilers for thermal power plants, a large apparatus for treating an exhaust gas, such as a denitrating apparatus is installed. FIG. 7 is an illustration of a structure of a device in the vicinity of a denitrating device installed in a duct used for an exhaust gas generated from such a combustion apparatus as described above, including an enlarged illustration of a part of the device. This apparatus comprises inlet duct 1 through which exhaust gas G is flowed in, catalyst duct 9 in which denitrating catalyst layer 4 is disposed, bend portion 2 which connects the inlet duct 1 and the catalyst duct 9, baffle lattice 3 disposed at the inlet of the catalyst duct 9 in the direction of the cross section of the duct, and beams 6 which directly support the baffle lattice 3. In this apparatus, exhaust gas G reaches, through inlet duct 1, to bend portion 2 formed at a midway of an exhaust gas duct, changes there its flow direction by 90°, flows into catalyst layer 4 disposed in catalyst duct 9 through baffle lattice 3, and then reaches to outlet duct 5 after subjected to a denitration. In the FIG. 7, the distribution of gas flow rates at the inlet of catalyst layer 4 is shown, and the distribution indicates that the flow rates of an exhaust gas are nonuniform in the direction of the cross section of the catalyst duct 9. Accordingly, it has been strongly desired or requested that
(i) an exhaust gas flows into catalyst layer 4 as uniformly as possible and
(ii) the flow of an exhaust gas which moves into catalyst layer 4 and the flow of the exhaust gas in the catalyst layer 4 become the same as much as possible, namely, the pattern of the flow passages of an exhaust gas immediately prior to flowing into catalyst layer 4 agrees with the pattern of the flow passages formed in the catalyst layer 4 for the exhaust gas. These desires are produced based on the following reasons:
That is, (i) the reason why the flowing of an exhaust gas into catalyst layer 4 is desired to be as uniform as possible is to efficiently increase the area in which the exhaust gas contacts with the catalyst in the catalyst layer 4. If the flowing of the exhaust gas into the catalyst layer was not uniform, specific portions of the catalyst in the catalyst layer 4 contact with a large amount of the exhaust gas and thus the performances of the catalyst decrease as a whole. Besides, there exists such a problem as the portions of the catalyst which contact with a large amount of the exhaust gas remarkably deteriorate and thus the life of the catalyst becomes short.
Next, (ii) the reason why the flow of an exhaust gas moving into catalyst layer 4 is desired to agree with the flow of the exhaust gas in the catalyst layer 4 as much as possible is that, for instance, when dusts such as fly ashes and unburnt matters are contained in the exhaust gas, the exhaust gas sometimes obliquely flows into flow passages formed among catalysts (catalyst plates) in the catalyst layer for the exhaust gas, and thus (a) the dusts directly collide against the catalyst plates to wear their surfaces and (b) the dusts adhere to and deposit on the front portions of the catalyst plates to impede the flow of the succeeding exhaust gas into the flow passages.
In order to resolve the problems described in (i) and (ii) above, a method in which baffle lattice 3 is disposed at the inlet of a catalyst duct (Japanese Patent No. 2637119) and another method in which a baffle device having baffle plates formed both at the upper stream side (on the upper flange) and the down stream side (on the lower flange) of a beam is provided at the inlet of a reactor (Laid open Japanese Utility Model Publication No. Sho 59-102134) are proposed.